This disclosure relates to a fuel supply system for an internal combustion engine (engine). The fuel supply system has a fuel pump for pumping fuel from a fuel tank to the engine, a motor for operating the fuel pump, and a control unit for performing feedback control of the duty ratio (also referred to as “duty cycle”) of electric pressure (also referred to as “voltage”) applied to the motor such that a fuel pressure comes close to a target fuel pressure.
For example, in a recent vehicle, fuel is pumped from a fuel pump into a fuel pipe and then is injected from injectors into an engine in order to supply the fuel to the engine. Further, feedback control of pressure of the fuel discharged from the fuel pump is performed such that fuel pressure in the fuel pipe increases or decreases depending on, e.g., an operating condition of the engine in order to further improve fuel efficiency, etc. However, in the state that the feedback control of the fuel pressure is performed, overshoot and undershoot of the fuel pressure are likely to occur, so that it is necessary to prevent such overshoot and undershoot.
In the fuel supply system for the engine according to Japanese Laid-Open Patent Publication No. 2007-285272, during a fuel cut condition, the overshoot of the fuel pressure in the fuel pipe is prevented by performing feedback control of a fuel pump, in which it is integrated with using an integration correction factor smaller than one is used in a condition without fuel cut. Further, during a return process from the fuel cut condition, the undershoot of the fuel pressure in the fuel pipe is prevented by temporarily increasing electric pressure applied to the fuel pump depending on the fuel pressure in the fuel pipe during the return process. In the invention disclosed in Japanese Laid-Open Patent Publication No. 2007-285272, prevention of the overshoot is effectively performed only in the condition that the operating condition of the engine is fuel cut, and prevention of the undershoot is effectively performed only in the condition that the operating condition of the engine is in the return process from the fuel cut condition. Accordingly, there has been a need for improved fuel supply systems.